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Member
Posts: 29
Location: Greensboro N.C. | I am getting some nasty tail rotor vibes at certain R.P.M.s.
seems to only hit at a certain speed then smooths out, but then when it is winding down,it hits that spot again.
is this normal? any ideas?
hope to test fly sunday. |
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Posts: 24
| Stu is up to his ear bones in electrical. I'll have him reply ASAP he comes in and will let him know now.
Did you test fly on Sunday? What was the result? I'll be on line this am, Tues, 6/15 |
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Member
Posts: 38
| Not sure this applies to the sngl seat Helicom, but while trying to balance the tail rotor, I discovered that there is a "Critical Speed" somewhere between 1600 and 2700 rpm. This showed itself via a 180 degree phase shift in the balance readings when only rpm was varied.
I also found that if I tried to balance the tail rotor when the ship was setting on concrete or asphalt, I had weird readings. It was only when I either put the ship on grass or vibration absorbing pads, that I could get stable balance readings. Another indication of some resonance between the airframe and the tail rotor. During all run ups or spool downs on concrete or asphalt, I always pull a little pitch (12" mp) when going thru the area of 400 rotor rpm. Once I was thru that area, I could reduce the collective and continue to 500rpm rotor.
I watched a Safari do a dance on concrete when the driver didn't pull any pitch. The Safari actually shook and bounced and changed heading by around 90 degrees on the pad and produced some dents in the fuel tanks.
One very bad design element common to this series is to drive the tail rotor drive shaft and the tail rotor at the same speed as the engine. Any resonant conditions propagate all over the place. This is common knowledge exhibited by most professional helicopter designers. At least one Safari owner that I have heard of changed the gear ratio in his tail rotor gear box. I've been told that the Jet Ranger drives it's tail rotor drive shaft at 6,000 rpm then reduces it at the tail rotor gear box.
Others have found a strong vibration doing agressive quick stops that reduced when a pair of 8' long brace tubes were added from the lower rear skid legs to the tail boom. In other words they increased the relative stiffness of the tail boom. (Bruce Belfield of NZ). On the other hand, the tail boom of the Bell 47 obviously resonates at a noticeably lower frequency than my Safari tail boom. I'm guessing but that might account for the yellow zone on the Bell 47 tach at around 1700 engine rpm.
In discussions with Larry Meidell, who is a known expert in helicopter vibrations, I told him that in the air, my Safari had a violent vibration at around 525 rotor rpm. When he was told the normal flight speed was 500rpm, his response was that the resonant frequency occurring at 525 was much too close. He recommended to keep the normal speed more than 20% away from any resonances. Using Larry's recommendation, the resonant frequency I observed needed to be moved to around 625. Maybe that is what the tail boom braces were doing. Stiffening up while keeping the mass about the same raises the resonant frequency. For the numbers of 525 and 625, this relates to a need to increase the stiffness by about 40%. One way to relate to this is to introduce a force to deflect the tail boom an amount X. The increase of 40% stiffness should decrease the X by 40% for the same force. Of course there is another way and that is to reduce the stiffness by 40%. Maybe this was the Bell approach?
Sorry for the long winded response but remember:
Ray Prouty advised young engineers seeking job security to go into the field of helicopter vibrations. |
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Member
Posts: 29
Location: Greensboro N.C. | thanks for the Reply, for some reason I never get an E-mail saying someone has replyed. vibrations where to bad to test fly. still working some bugs out.
got some minor damage during a run up, I think my test pilot held it in the resonant speed area to long. I can honestly say I am not having a warm fuzzy feeling for the 2 seat Commuter as much now.
got to be away to get ride of this problem. I wonder what other ratio gear box would work.
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Posts: 12
| Reid West had a single seat ,Called it the wedge,It had all sorts of vibes all over ,some due to thr rotor system,some due to the power plant it took a few months to sort out the helicopter.But when it was sorted out flew like a tank. The solid rotor shaft and the funky head and controls didn't help. |
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Member
Posts: 29
Location: Greensboro N.C. | well I have learned not to take peoples word for things once again. the guy I bought the H-1b from said he had been hovering it before he stored it.
there is no way he hovered this helicopter! I had taken his word for it and never checked the TR balance. won't do that again. turns out one blade was 2 OZ. heavyer then the other. I am also going to check the run out on the TR shaft to make sure it has not gotten bent.
the TR blades are stainless steel with a 4130 round tube spar in them. I am thinking of making some lighter weight TR blades for it or trying some extruded TR blades from Vortech. maybe I am wrong but a TR blade that weighs 1 pound and 4 OZ. is just to heavy.
I have heard that several Commuters have had tail rotor failures in the past, I can help but think that heavy a tail rotor blade canm not be a good thing for it.
what are ya'll thoughts on this, I have a guy I talk to in Romania that has a sweet Commuter 2B and he has a R-22 tail rotor on his.
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Posts: 38
| Sorry for being so slow on reply but my bucket has been exceptionally full lately.
Wit the vibrations you are reporting you had better check closely the tail rotor spindle. Several have been found with inadequate fillets where the diameter changes for bearing interfaces. These have all been the titanium ones tho.
Several people have reported postive experiences with lighter tail rotor blades. Aluminum and titanium have been used and Bruce Belfield in NZ has a pair of carbon fiber blades that he is going to test on his jig...
I believe that Mark Richards of CHR told me that he had a tail boom strike using lighter blades??? I know that at the time he supported the SS blades.
The Safari tends to be tail heavy and lighter weight quality blades would be an improvement. |
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Posts: 29
Location: Greensboro N.C. | Well I have learned 1 thing, who ever put the steel spar in it and the alum. plug into the blade did not mean for it to ever come out.
one spacer is longer then the other and that seems to be where my off balance is coming from. I put the blade in a 20 ton bearing press and it started to bow the 5/16 steel rod instead of pushing the plug out.
I guess my next thing to try will be to drill out the plug till they weigh the same.
I was going to try the extruded aluminum blades from Vortech but they are 2" shorter each blade,so I have concerns about losing 4" of tail rotor disk.
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Member
Posts: 29
Location: Greensboro N.C. | well under closer inspection while trying to even out the weight of each blade I have found the problem.
the steel tube spar in the heavier Blade ,it turns out is made from a section of thicker wall tubing.
so I guess all I can do now is add some weight to the lighter blade.
I may still try and make some aluminum blades to try.
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Member
Posts: 29
Location: Greensboro N.C. | Well Saturday,I spent the day balancing the tail rotor blades and hub. hopefully it will be a lot smoother on the next run up.
still got this bird for sale, but will continue to play with it and get it dialed in better as I go along.
it did feel good to walk into the shop and see my baby back together. |
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Tail rotor vibes- older single seat Helicom Commuter.
